High data-rate wireless access is demanded by many applications. Traditionally, more bandwidth is required for higher data-rate transmission. However, due to spectra limitations, techniques for increasing bandwidth are often impractical and/or expensive. In one known system, multiple transmit and receive antennas are used to obtain transmit diversity or to form multiple-input multiple-output (MIMO) channels. Multiple transmit antennas have also been used to provide diversity in wireless systems. Transmit diversity can be based on linear transforms or by space-time coding. Space-time coding in particular is characterized by high code efficiency and can improve the efficiency and performance of Orthogonal Frequency Division Multiplexing (OFDM) systems. The system capacity can be further improved if multiples transmit and receive antennas are used to form MIMO channels. Compared with a signal-input single-OUTPUT (SISO) system with flat Rayleigh fading or narrowband channels, a MIMO system can improve the capacity by a factor of the minimum of the number of transmit and received antennas.
FIG. 1 shows a conventional OFDM system 10 including subsystems for transmission and reception of data. A coding subsystem 12 encodes binary data from a data source. The coded data is interleaved by an interleaving subsystem 14 and then mapped onto multi-amplitude multi-phase constellation symbols by a mapping subsystem 16. In one particular embodiment, the multi-amplitude multi-phase constellation symbols include quadrature phase shift keying (QPSK) symbols. Pilot signals can then inserted by a pilot insertion subsystem 18 to estimate the channel at the remote subscriber unit receivers. A serial-to-parallel conversion subsystem 20 converts the serial data stream to a parallel data stream that is provided to an inverse fast Fourier transform (IFFT) subsystem 22.
The transformed data is converted to serial data stream by a parallel-to-serial converter 24. Cyclic extension and windowing can be added by a subsystem 26 prior to digital-to-analog conversion by a DAC 28 and transmission by an antenna system 30. The receive portion 32 of the OFDM system includes similar corresponding components for extracting the data from the received OFDM signal.
As shown in FIG. 2, the known OFDM system 10 utilizes an overlapping orthogonal multicarrier modulation technique having a plurality of subcarriers 50. FIG. 3 shows the orthogonal nature of the subcarriers. More particularly, each of four subcarriers 60 of one OFDM data symbol has an integral number of cycles in the interval T. The number of cycles between adjacent subcarriers differs by one.
In one known OFDM transmission system, the complexity of the space-time processor increases as the bandwidth increases and the performance of the space-time processor is significantly degraded when estimated channel parameters are used to construct space-time processors.
It would, therefore, be desirable to provide a MIMO OFDM system having enhanced signal detection. It would further be desirable to increase the accuracy of channel parameter estimates.